1. Field of the Invention
The present invention relates to a control apparatus and method for a brushless DC motor.
2. Description of Related Art
In a conventional DC motor, commutations, essentially a mechanical switching operation, control the current through the stator windings. This operation is accomplished in conventional DC motors with brushes and segmented commutators. In such a construction, the brushes wear and require frequent replacement. Sparking and its attendant generation of RF noise cannot be avoided. However, DC motors typically have higher torque and efficiency than AC induction motors. Thus, recently, a brushless DC motor has been developed. Some brushless motors have one or more sensors, for example hall effect devices, to detect the position of a permanent magnet rotor of the DC motor without touching the rotor. Furthermore, sensorless brushless DC motors have been developed which are simpler and less expensive than brushless DC motors with sensors.
A typical sensorless brushless DC motor has three phase stator windings and a permanent magnet rotor. The stator windings are selectively energized or caused to conduct current to apply a rotating magnetic field to the permanent magnet rotor. In the energization pattern, at any particular time only two of the phases are conducting current, leaving one phase not conducting current. The stator windings are energized and deenergized during the rotation of the brushless DC motor, based on a comparison of the induced voltage generated in the non-conducting stator winding with a specified reference voltage. In this motor, three comparator circuits are provided to compare the induced voltage of each winding and the specified reference voltage.
In this type of brushless DC motor, an activation malfunction or a short-circuit of a switching transistor of an inverter, which functions as a static commutator, is detected based on the current passing through the stator windings or the motor current. For detection of these malfunctions, the DC motor current is detected by a current detector inserted in series with the current providing circuit of the DC motor. When the detected current is above a specified value, it is determined that a switching transistor of the inverter has short-circuited or the motor failed to start rotating. Then, power is disconnected from the DC motor.
As described above, the induced voltages generated in the non-conducting windings during rotation of the brushless DC motor are compared with a reference voltage. The position of the permanent magnet rotor is detected based on that variation. Sequential current conduction phase switching is executed taking this detected rotor position point as a reference. Since the rotor is not rotating right after the motor is energized, induced voltages are not generated in the non-conducting stator windings because there are no rotating magnetic fields. Therefore, on energization, the stator windings are selectively energized without detection of the rotating position of the rotor in a process called "forced commutation". Then, when a specified time has elapsed after which it is possible to detect the induced voltages generated as a result of forced commutation, commutation based on the detected rotor position starts.
However, when the motor rotor is mechanically constrained, or a drive circuit which drives the switching transistors of the inverter does not operate normally, the motor cannot rotate. Thus, the rotor position cannot be detected even after the specified time has elapsed. At this point, commutation is suspended, and an abnormality, in the form of an activation malfunction, is signaled.
In the above brushless DC motor, the following three abnormalities are considered to have the highest probability:
(a) The motor fails to rotate due to a mechanical failure locking the position of the motor rotor. PA1 (b) Open-phase output due to a failure in the inverter or the drive circuit. PA1 (c) Rotor position cannot be detected due to a failure of an element in a rotor position detection circuit which includes the comparator circuits.
These failures were all expressed as activation malfunctions in the prior art. However, when repairing these failures, the motor must be replaced in response to malfunction (a); the inverter element or drive circuit must be replaced or the wiring between these circuits must be checked in response to malfunction (b); and the position detection circuit must be replaced in response to malfunction (c). Thus, after a failure, the service engineer must discriminate which of these three malfunctions is the cause.
Usually, the circuit components for a DC motor, that is, the inverter, the drive circuit and the position detection circuit, are all mounted on a single substrate. In such a DC motor, when the malfunction is caused by circuit parts, the DC motor can be repaired by replacing that substrate, so that discrimination between malfunctions (b) and (c) is not required. Therefore, it is only necessary to discriminate malfunction (a) from malfunctions (b) and (c).
However, in the prior art there was no device which could discriminate between rotation failure due to motor malfunction (a) and malfunctions (b) and (c). Thus, after an activation malfunction was detected, the service engineer replaced one of the parts of the DC motor. If the DC motor then performed normally, the replaced part was considered faulty. If the activation malfunction re-occurred despite the replacement, one of the unreplaced parts might be faulty. Therefore, the service engineer reinstalled the previously replaced part and replaced one of the unreplaced parts with a new component until the DC motor operated normally. Thus, the work of the service engineer was very inefficient.
Furthermore, if the DC motor was built into a sealed compressor to rotate a compression mechanism, pipes connected to the compressor to carry refrigerant had to be disconnected. Then the compressor itself had to be replaced. Therefore, it is desirable to accurately determine the cause of a failure.